Matthew Royal

Chip scale integrated microresonator sensing systems.

Medicine, environmental monitoring, and security are application areas for miniaturized, portable sensing systems. The emerging integration of sensors with other components (electronic, photonic, fluidic) is moving sensing toward higher levels of portability through the realization of self-contained chip scale sensing systems. Planar optical sensors, and in particular, microresonator sensors, are attractive components for chip scale integrated sensing systems because they are small, have high sensitivity, can be surface customized, and can be integrated singly or in arrays in a planar format with other components using conventional semiconductor fabrication technologies. This paper will focus on the progress and prospects for the integration of microresonator sensors at the chip scale with photonic input/output components and with sample preparation microfluidics, toward self-contained, portable sensing systems.

Progress in Chip-Scale Photonic Sensing

Chip-scale integrated planar photonic sensing systems for portable diagnostics and monitoring are emerging, as photonic components are integrated into systems with silicon (Si), Si complementary metal-oxide semiconductor, and fluidics. This paper reviews progress in these areas. Medical and environmental applications, candidate photonic sensors, integration methodologies, integrated subsystem demonstrations, and challenges facing this emerging field are discussed in this paper.

Chip Scale Optical Microresonator Sensors Integrated With Embedded Thin Film Photodetectors on Electrowetting Digital Microfluidics Platforms

Miniaturized, portable, sensitive, and low cost sensing systems are important for medical and environmental diagnostic and monitoring applications. Chip scale integrated photonic sensing systems that combine optical, electrical, and fluidic functions are especially attractive for sensing applications due to the high sensitivity of optical sensors, the small form-factor of chip scale systems, and the low-cost processing possible for systems fabricated with well-developed mass production techniques. In this paper, a chip scale sensing system, which is composed of a planar integrated optical microdisk resonator and a thin film InGaAs photodetector, is integrated with a digital microfluidic system. This system was designed, fabricated, and experimentally characterized by dispensing and moving droplets of glucose solution from the reservoir to the microresonator sensor. The optical output of the resonator was transduced by the integrated photodetector to an electrical current signal for readout.

Voltage switching of a VO2 memory metasurface using ionic gel

We demonstrate an electrolyte-based voltage tunable vanadium dioxide (VO2) memory metasurface. Large spatial scale, low voltage, non-volatile switching of terahertz (THz) metasurface resonances is achieved through voltage application using an ionic gel to drive the insulator-to-metal transition in an underlying VO2 layer. Positive and negative voltage application can selectively tune the metasurface resonance into the “off” or “on” state by pushing the VO2 into a more conductive or insulating regime respectively. Compared to graphene based control devices, the relatively long saturation time of resonance modification in VO2 based devices suggests that this voltage-induced switching originates primarily from electrochemical effects related to oxygen migration across the electrolyte–VO2 interface.

Droplet-Based Sensing: Optical Microresonator Sensors Embedded in Digital Electrowetting Microfluidics Systems

Electrowetting-on-dielectric (EWD) microfluidics is an emerging platform for practical applications such as water quality testing and medical diagnostics. Low power consumption, low sample and reagent volumes, small size, and rapid fluid transport are features of electrowetting microfluidic platforms that will enable the development of cost-effective, rapid time-to-result, and portable point-of-care diagnostic devices. Microresonator sensors are an excellent sensor technology for integration into these microfluidic systems, because they perform high sensitivity detection of proteins, DNA, and other biologically relevant molecules while tolerating a droplet oil encapsulation layer. This paper reports on a SU-8 polymer microresonator embedded in the top plate of the EWD system, which enables addressing of the sensor with a single droplet of in volume and enables droplets to be moved onto and off of the sensor. This system is the first to demonstrate actuation of droplets onto and off of an integrated microresonator sensor. Both photolithographically patterned and electron beam lithographically patterned microresonator sensors were tested, and the effect of a conventional filler medium, silicone oil, on the sensor sensitivity was investigated.

Integrated Sample Preparation and Sensing: Polymer Microresonator Sensors Embedded in Digital Electrowetting Microfluidic Systems

Portable point-of-care (POC) medical diagnostic devices demand low power consumption, efficient use of low sample and reagent volumes, and small size. A “digital” electrowetting-on-dielectric (EWD) microfluidics system with integrated optical sensors is a promising platform for portable POC diagnostic devices that integrate sample preparation with sensing. Herein is reported the highest sensitivity and figure of merit (FOM) microresonator sensor integrated with an EWD system to date [FOM is the product of the quality (Q) factor and the sensitivity (S)]. The EWD system embedded microresonator sensor had a measured FOM of 0.60 × 106 nm/RIU (Q = 8,400, S = 72 nm/RIU) in 2-cSt silicone oil and water, which is nearly double the best FOM previously reported for an microresonator placed on top of an EWD system. Additionally, a nominal FOM of 1.2 × 106 nm/RIU (Q = 15 000, S = 82 nm/RIU) was measured for the vertically coupled sensor fabricated on a standard SiO2/Si substrate. This FOM is the highest reported to date for SU-8 microresonators probed at wavelengths around 1550 nm. These results indicate that high-performance polymer sensors can be integrated with low-power-consumption EWD microfluidics sample preparation systems toward the development of portable POC diagnostic devices.

Two-dimensional reconfigurable gradient index memory metasurface

Creation and control of spatial gradients in electromagnetic properties is a central theme underlying optical device design. In this work, we demonstrate that through modification of the spatial and temporal distribution of current, we can obtain increased control over the shape of these gradients. We are able to write spatially sharp gradients with ∼50% change in the index of refraction over length scales of only a few wavelengths as observed through diffraction limited terahertz spectroscopy. Furthermore, we assess the potentials for such gradients for beam-steering applications.

Fluid Transport in Partially Shielded Electrowetting on Dielectric Digital Microfluidic Devices

Theoretical and experimental approaches verifying the fluidic operation of a partially shielded digital microfluidics device are presented in this paper. This paper is motivated by recent demand from the synthetic biology community for electro-wetting on dielectric (EWD) enabled in-droplet electroporation, but is generalizable to a range of EWD applications that require shielding structures to be patterned on the EWD. An electrode patterned in an additional metal layer on the insulator that supports EWD actuation reduces the effective strength of the EW force due to dielectric shielding at the droplet contact line. A numerical model was developed to predict the impact of the partially shielding electrode on threshold voltage, EW force, fluid velocity, and droplet transport time. Compared with a batch of devices lacking the extra electrode, the presence of the added metal layer resulted in a 29% increase in threshold voltage, an 82% increase in transport time, and a 44% decrease in average transport velocity. Each trend agrees with the simulation results obtained from the fluid transport model. These results support the development of design rules for microfluidic devices that require partially shielding metal layers to integrate with EWD device architectures.

Scalable Device for Automated Microbial Electroporation in a Digital Microfluidic Platform

Electrowetting-on-dielectric (EWD) digital microfluidic laboratory-on-a-chip platforms demonstrate excellent performance in automating labor-intensive protocols. When coupled with an on-chip electroporation capability, these systems hold promise for streamlining cumbersome processes such as multiplex automated genome engineering (MAGE). We integrated a single Ti:Au electroporation electrode into an otherwise standard parallel-plate EWD geometry to enable high-efficiency transformation of Escherichia coli with reporter plasmid DNA in a 200 nL droplet. Test devices exhibited robust operation with more than 10 transformation experiments performed per device without cross-contamination or failure. Despite intrinsic electric-field nonuniformity present in the EP/EWD device, the peak on-chip transformation efficiency was measured to be 8.6 ± 1.0 × 108 cfu·μg-1 for an average applied electric field strength of 2.25 ± 0.50 kV·mm-1. Cell survival and transformation fractions at this electroporation pulse strength were found to be 1.5 ± 0.3 and 2.3 ± 0.1%, respectively. Our work expands the EWD toolkit to include on-chip microbial electroporation and opens the possibility of scaling advanced genome engineering methods, like MAGE, into the submicroliter regime.

Arbitrary birefringent metamaterials for holographic optics at λ = 1.55 μm

This paper presents an optical element capable of multiplexing two diffraction patterns for two orthogonal linear polarizations, based on the use of non-resonant metamaterial cross elements. The metamaterial cross elements provide unique building blocks for engineering arbitrary birefringence. As a proof-of-concept demonstration, we present the design and experimental characterization of a polarization multiplexed blazed diffraction grating and a polarization multiplexed computer-generated hologram, for the telecommunication wavelength of λ = 1.55 μm. A quantitative study of the polarization multiplexed grating reveals that this approach yields a very large polarization contrast ratio. The results show that metamaterials can form the basis for a versatile and compact platform useful in the design of multi-functional photonic devices.